Citron-N is a neuronal Rho-associated protein involved in Golgi organization through actin cytoskeleton regulation

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Abstract
The actin cytoskeleton is best known for its role during cellular morphogenesis. However, other evidence suggests that actin is also crucial for the organization and dynamics of membrane organelles such as endosomes and the Golgi complex1,2,3. As in morphogenesis, the Rho family of small GTPases are key mediators of organelle actin-driven events4,5, although it is unclear how these ubiquitously distributed proteins are activated to regulate actin dynamics in an organelle-specific manner. Here we show that the brain-specific Rho-binding protein Citron-N6 is enriched at, and associates with, the Golgi apparatus of hippocampal neurons in culture. Suppression of the whole protein or expression of a mutant form lacking the Rho-binding activity results in dispersion of the Golgi apparatus. In contrast, high intracellular levels induce localized accumulation of RhoA and filamentous actin, protecting the Golgi from the rupture normally produced by actin depolymerization. Biochemical and functional analyses indicate that Citron-N controls actin locally by assembling together the Rho effector ROCK-II and the actin-binding, neuron-specific, protein Profilin-IIa (PIIa). Together with recent data on endosomal dynamics5, our results highlight the importance of organelle-specific Rho modulators for actin-dependent organelle organization and dynamics.